(i) Field of the Invention:
This invention relates to a write-once optical recording medium using a converged beam of semiconductor laser. This invention also relates to a process for the fabrication of such an optical recording medium productively by forming recording layers thereof by the coating method.
(ii) Description of the Prior Art:
As write-once optical recording media of the above-described sort, there have been proposed recording media having inorganic recording layers of thin films of low melting-point metals such as tellurium, tellurium alloys and bithmuth alloys and, as disclosed in U.S. Pat. No. 4,298,975 for example, recording media making use of phthalocyanine dyes as recording layers.
These recording media are low in productivity, since their recording layers have to be formed in vacuo by vacuum evaporation, sputtering or the like. Further, media having inorganic recording layers are limited in recording density due to the greater thermal conductivities of the recording layers. Further, these media are liable to deteriorate significantly from oxidation in the air. There is also a potential danger with respect to toxicity since they employ poisonous materials such as tellurium.
With a view toward providing a solution to the above-described problems, optical recording media with recording layers formed by coating soluble organic dyes have been proposed. For example, processes making use of the spin coating technique have been developed to coat organic dyes which are soluble in organic solvents and exhibit absorption in the oscillation wavelength range of semiconductor lasers, such as dithiol metal complexes, polymethine dyes, squarylium dyes and naphthoquinone dyes. Some of these processes have already been put into practice. Such conventional recording media, however, have poor durability as demonstrated by those containing, for example, cyanine or squarylium dyes as recording layers out of the dyes proposed to date. Further, they require additionally reflective thin layers made of inorganic materials such as metals or metal oxides for supplementing their reading function, because they have substantially low reflectances when provided solely with recording layers of dyes such as dithiol metal complexes.
For example, U.S. Pat. No. 4,492,750 relates to media which use alkyl-substituted naphthalocyanine dyes. It discloses an optical recording medium having reflective layer of a material such as Al provided on a glass or polymethyl methacrylate substrate and a layer of an optical recording composition provided on the reflective layer and containing vapor-treated particles of an alkyl-substituted naphthalocyanine dye, the particle sizes of which range from 0.005 .mu.m to 0.1 .mu.m, dispersed in a polymeric binder. As disclosed in the above U.S. Patent, the optical recording layer cannot be formed directly on the substrate and the reflective layer made of an inorganic material such as Al must be formed additionally on the substrate by a vacuum process such as vacuum evaporation. The fabrication process of the optical recording medium is thus rather complicated.
In addition, the above optical recording medium is accompanied by a more serious problem. A recording layer making use of an organic dye has an inherent feature of low thermal conductivity. Hence, it is potentially expected to exhibit high recording sensitivity. When a reflective layer made of a metal or inorganic material having high thermal conductivity is provided, the thermal energy produced by a writing laser beam irradiated onto the recording layer is however caused to dissipate through the reflective metal layer due to the high thermal conductivity of the reflective metal layer, so that the thermal energy is not effectively used for the formation of pits and holes. As a result, the recording sensitivity is reduced considerably.
In the case that a reflective layer made of an inorganic material such as Al is provided, when a laser beam is irradiated through the substrate for recording signals or reading them out, the laser beam is not allowed to reach the recording layer even if the substrate itself is transparent. This is obvious because the laser beam is shut off by the reflective layer of the metal or inorganic material which practically prevents transmission of light therethrough. Whenever such a reflective layer is provided, it is naturally impossible to perform the recording and reading-out of signals through the associated substrate. Accordingly, the recording and reading-out of signals have to be conducted on the side of the recording layer.
In this case, the slightest existence of dust or scratches on the surface of the recording layer results in considerable disturbance to the accurate recording and reading-out of signals which take the form of pits and holes. For practical application, the above-mentioned optical recording medium thus requires a dust protective layer as an overcoat on the recording layer. If it becomes feasible to conduct the recording and reading-out of signals by means of a laser beam through a transparent substrate, such a dust protective layer will not be required at all. It is because the existence of dust or scratches on the medium surface on the incident side of the laser beam, where the laser beam is still un-focused, has no effect on recording and reading-out of the signals, due to the distance corresponding to the thickness of the substrate. As described above, optical recording media with reflective layers made of inorganic (metallic) materials such as Al are accompanied by numerous drawbacks.
On the other hand, in Japanese Patent Laid-Open No. 112794/'83, there is proposed what is called a function-separating recording film formed by laminating a high-reflective dye layer which is not caused to change by a laser beam and a layer of a light-absorptive organic substance.
Further, Japanese Patent Laid-Open No. 224448/'83 relates to a function-separating recording layer formed by laminating a reflective layer comprising a bronze-lustrous organic substance or low melting point metal and an absorptive layer composed of an organic substance. It proposes a stable recording layer making a good use of the characteristics cf an organic dye and superb in recording and reading-out of signals.
In Japanese Patent Laid-Open No. 239948/'85, a proposal is made to use a cyanine dye or merocyanine dye as a reflective layer and laminate thereon an absorptive layer, thereby preparing an optical information recording medium which protects the reflective layer from being affected by the surrounding atmosphere.
These inventions are based on the technological concept that a reflective layer is practically laminated on a substrate so that focal point control and reading-out of recording signals are effected by the reflected rays at the reflective layer and the boundary of the substrate. However, in this method, as the amount of reflected ray increases at the boundary of the substrate, the amount of the laser beam reaching the absorptive layer is decreased and the sensitivity is thus reduced. Further, when a cyanine dye or merocyanine dye is used as the reflective layer, it is apt to be deteriorated by the laser beam used for reading-out. Thus, it has a fatal defect in longterm use and also is liable to be deteriorated by room light.
The means for lamination proposed in Japanese Patent Laid-Open Nos. 112794/'83 and 224448/'83 are too disadvantageous in productivity and cost to put into practice. In the case of lamination of organic dyes by the evaporation-evaporation method or evaporation-coating method, dyes useful in the evaporation method are inherently very limited and therefore dyes usable in this method are restricted to those having specific molecular structures. Moreover, these optical recording media are fabricated with substantially the same complexity as in the formation of reflective layers comprising inorganic compounds, and hence their productivity is lowered.
On the other hand, in the case of lamination by the coating-coating method, no such defects are observed. However, it becomes an important subject not to give damage to the substrate being coated. It is because that substrates for optical recording media generally have pregrooves with a pitch of several micrometers for an optical beam to conduct recording and reading-out of signals and also address signals and control signals called prepits in the form of minute pits and holes, and the deterioration of these pregrooves and prepits has to be absolutely avoided. Further, when a recording layer of a laminated structure is formed by the coating-coating method, there is a subject that no deterioration must be caused in the first layer and the above-described pregrooves and prepits. Furthermore, no proposal has been made to date to obtain necessary information for this sort of system control securely.
In view of the foregoing technical background, there has been an urgent need to develop optical recording media which are highly sensitive in recording, are stable to a reading light or room light, and are capable of obtaining system control signals securely.